feat: Add R program to find longest substring without repeating characters

[sgindeed]

1. Prompts the user to enter a string.
2. Converts the string to lowercase (optional, for case-insensitive processing).
3. Splits the string into individual characters.
4. Uses a hash table to track the last seen index of each character.
5. Implements a sliding window to find substrings without repeating characters.
6. Computes and updates the length of the longest substring found.
7. Displays the length of the longest substring without repeating characters.

[Copilot]

Pull Request Overview

This PR adds an R program that finds the length of the longest substring without repeating characters using a sliding window algorithm. The program prompts for user input, processes the string character by character, and uses a hash table to track character positions.

• Implements sliding window algorithm with hash table for character tracking
• Includes user input handling and case-insensitive processing
• Provides clear output showing the longest substring length

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[siriak]

Looks good, thanks!